President Viktor Yushchenko has said he cannot consider a proposal to make the man he defeated in the 'Orange Revolution' of 2004 prime minister as such a move would be unconstitutional.
Mr Yushchenko, in a statement issued as parliament descended into chaos for its second straight sitting, said a new prospective coalition backing Moscow-friendly Viktor Yanukovich as prime minister had been formed too quickly.
The appeal said any action undertaken by the coalition "including any proposal submitted to the president ... cannot be examined as the rules of the constitution and parliamentary procedures were not observed".
But Mr Yanukovich, who made a comeback this year after being humiliated in the revolution, said there could be no compromise on his candidature for premier. He and his colleagues insisted the president endorse him under constitutional procedures.
Liberal "orange" parties, who failed in three months of talks after a March election to produce a coalition, demanded the president dissolve parliament and call a new poll.
Ukraine has been pitched into turmoil since March, with no working parliament and no full-fledged government. The European Union, whose membership is the ultimate goal of the pro-Western Yushchenko, has expressed dismay at the turmoil.
Talks on putting together an "orange" team collapsed when one of its members, the Socialist Party, defected last week.
An alternative grouping, led by Yanukovich's Regions Party and including Socialists and Communists, was formed in its place in less than 24 hours and proposed him as prime minister.
Mr Yanukovich said that if the president failed to endorse him, deputies would invoke other rules to do it themselves.
The president has given politicians until the end of July to produce a government or face dissolution.